The present invention relates to a method of producing a waterproof, gas-permeable composite sheet, and more particularly to a method of producing a waterproof, gas-permeable composite sheet having not only good softness but also high mechanical strength and desirably high heat resistance and stretchability.
Sheets and films having good waterproofness and gas permeability have come to be widely used as leak-preventive sheets for disposable diapers, etc., in recent years.
Such gas-permeable films are generally produced by blending polyolefin resins with inorganic fillers, melting and forming them into films and then stretching them uniaxially or biaxially to provide them with micropores. In such gas-permeable films, extremely small fractures are produced at points of inorganic fillers by stretching, resulting in the formation of gas-permeable pores of 1-4 .mu.m or so.
However, such gas-permeable films do not have sufficient gas permeability, and because of their rigidity inherent in stretched polyolefin films, they give paper-like touches so that they are not suitable for applications requiring softness.
It was also proposed to produce a soft, porous film by adding to polyolefin resins inorganic fillers and low-melting point polymers of the same types, rubbery polymers, olefinic thermoplastic elastomers, etc. However, the resulting films are still insufficient in gas permeability and poor in strength.
Therefore, various proposals to provide softness to stretched gas-permeable films made of polyolefin resins and inorganic fillers. For instance, Japanese Patent Laid-Open No. 60-257221 discloses a method of producing a soft, porous film by extruding in a molten state a composition comprising 100 parts by weight of a polyolefin resin, 25-400 parts by weight of a filler, and 1-100 parts by weight of a liquid or waxy hydrocarbon polymer or a mixture of the hydrocarbon polymer and an epoxy group-containing organic compound, and then stretching the resulting film biaxially.
Further, Japanese Patent Laid-Open No. 62-10141 discloses a method of producing a porous film or sheet by forming a film or sheet from a composition comprising a polyolefin resin, a filler and a triglyceride in a molten state, and stretching the film or sheet.
Further, Japanese Patent Laid-Open No. 62-27438 discloses a method of producing a gas-permeable film by at least uniaxially stretching a film made of a composition comprising 42-87 volume % of a polyolefin resin and 58-13 volume % of an inorganic filler, the polyolefin resin being a mixture of 50-95 weight % of linear low-density polyethylene and 50-5 weight % of branched low-density polyethylene, and the composition containing 3-25 parts by weight, per 100 parts by weight of the composition, of an aliphatic alcohol-aliphatic acid ester which is a compound of aliphatic acid having 10-22 carbon atoms and aliphatic alcohol having 1-12 carbon atoms.
Further, Japanese Patent Laid-Open No. 58-149303 discloses a microporous film as a liquid-impermeable, leak-proof sheet to be integrally laminated with an absorbant for disposable diapers, which is produced by blending a composition comprising 100 parts by weight of a polyolefin resin, 28-200 parts by weight of a filler and 10-70 parts by weight of a liquid or waxy polyhydroxy saturated hydrocarbon formed by the hydrogenation of liquid polybutadiene having hydroxide end groups, forming the composition into a film and then stretching it by 1.2 times or more at least uniaxially to provide it with micropores.
However, since any of the above gas-permeable films are produced by stretching, they are thin and so poor in mechanical strength. Accordingly, they are often used in combination with other materials. Thus, secondary processing is often needed, leading to the increase of their prices. In addition, since they do not contain rubber materials, they are poor in softness.
In view of these problems, the inventor previously found that a gas-permeable composite film having high mechanical strength and good softness can be obtained by hot-pressing a stretched film made of a composition comprising a crystalline polyolefin resin, a rubber polymer and a filler to a mesh-like sheet at a temperature equal to or higher than a temperature at which the heat shrinkage of the stretched film initiates, to adhere the stretched film to the mesh-like sheet and simultaneously causing the heat shrinkage of the stretched film, thereby making the stretched film microporous, and filed a patent application for such a gas-permeable composite film in Japan (Japanese Patent Application No. 62-170497).
This gas-permeable composite film has not only excellent softness but also excellent mechanical strength and gas permeability. However, recently further improved waterproofness, gas permeability and softness are required for sheets of disposable diapers, sports wear, etc. However, since the above composite film is based upon polyolefin, it is not necessarily satisfactory in softness for the above applications. In addition, in the above applications, good stretchability is often desired.
Moreover, recently, heat resistance has become an important requirement for gas-permeable composite films so that they are not burned or scorched even by direct contact with a lighted cigarette, etc. To improve the heat resistance of the composite films, it is preferable to cause the cross-linking of such components as an ethylene-propylene-diene copolymer and an ethylene-vinyl acetate copolymer in the films, but a simple heat treatment leads to the melting of the films so that the films lose their micropores. In addition, in the heat treatment process, unacceptable odor may be generated. It is possible to carry out the cross-linking of the films only by irradiating electron beam, but the irradiation of electron beam may cause molecular scission which in turn causes the generation of strong odor (acetic acid odor).